1. Field of the Invention
The present invention relates to a piston rod guide for axially slidably guiding a piston rod of a piston-cylinder unit.
2. Description of the Related Art
Prior art piston rod guides are known which are made from sintered materials. Replacements for the sintered material in piston rod guides for piston-cylinder units are increasingly being sought after, since the porosity of the sintered material requires that the piston rod guide be sealed over its entire surface with respect to the working chamber. For example, machined aluminum guides are in general use, but these guides are provided with a coating to maintain an acceptable wear performance. However, this coating has an adverse effect on the coefficients of friction.
Slide bushes have been used as a measure to counteract the less satisfactory coefficients of friction of the aluminum guide. There are two main designs of these slide bushes which differ in the position of the slide bush with respect to the piston rod seal. In a first design, the slide bush lies on a side of the piston rod seal away from the working chamber to be sealed. Although the structure of the piston rod guide is relatively simple, it is impossible for any entrained oil to reach the slide bush, and consequently the slide bush becomes worn relatively quickly, particularly in the event of transverse forces. A remedy to this is provided by an entrained oil duct, which guides controlled volumes of oil to the slide bush. However, the entrained oil duct in turn involves increased construction outlay. Reference is made, for example, to German reference DE 44 36 907 A1, which discloses a two-part piston rod guide made from sheet metal.
In principle, the fiction performance is better with a second design of piston rod guides in which the slide bush is arranged between the piston rod seal and the working chamber to be sealed. By way of example, German reference DE 42 07 099 A1 is cited, in which two annular shaped parts form a guide closure. One of the two annular shaped parts, in half section, has a U-shaped profile which forms an entrained oil duct. The second annular shaped part is designed as a multiple stepped cap and is in contact by means of an overlap in the area of the external diameters of the two annular shaped parts. There are two drawbacks associated with this design. Firstly, the guide bush is rather elastic in the transverse direction, and secondly the position of the piston rod seal is not clearly defined, since the cap of the second annular shaped part provides a bearing surface on only one axial side.
A modification to this design is described in German reference DE 42 07 053 A1, but this modification exhibits the same drawbacks described above.